1. Field of the Invention
The present invention relates generally to a method and apparatus for handover in a mobile communication system, and in particular, to a method and apparatus for reducing the data which is forwarded between base stations during handover in a mobile communication system.
2. Description of the Related Art
A Universal Mobile Telecommunication Service (UMTS) system, one of the mobile communication systems, is a 3rd generation (3G) asynchronous mobile communication system that uses Wideband Code Division Multiple Access (WCDMA), based on Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS) which are European mobile communication systems.
In 3rd Generation Partnership Project (3GPP) in charge of UMTS standardization, Long Term Evolution (LTE) is now under discussion as a next-generation mobile communication system of the UMTS system. LTE, expected to be commercialized in around 2010, is a technology for realizing high-speed packet based communication supporting a transfer rate of a maximum of about 100 Mbps. For this technology, a discussion is made on several schemes, including, for example, a scheme of reducing the number of nodes located in the communication path by simplifying a configuration of the network and a scheme of maximally approximating wireless protocols to wireless channels.
FIG. 1 is a diagram illustrating an example of a system configuration based on a UMTS system among the next-generation mobile communication systems.
Referring to FIG. 1, as illustrated, it is intended that Evolved Radio Access Networks (E-RANs) 110 and 112 are simplified to a 2-node configuration consisting of Evolved Node Bs (ENBs, or Node Bs) 120, 122, 124, 126 and 128, and Enhanced Gateway GPRS Support Node (EGGSNs) 130 and 132. A User Equipment (UE) 101 accesses an Internet Protocol (IP) network 114 by means of the E-RANs 110 and 112.
The ENBs 120 to 128, which correspond to the existing Node Bs of the UMTS system, are connected to the UE 101 by wireless channels. Compared with the existing Node Bs, the ENBs 120 to 128 perform more complicated functions. In LTE, since all user traffics including the real-time services such as Voice over IP (VoIP) are serviced over a shared channel, an apparatus is needed that gathers status information of UEs and performs scheduling based on the gathered information, and the ENBs 120 to 128 serve as the apparatus. Commonly, one ENB controls a plurality of cells. In order to realize the transfer rate of a maximum of 100 Mbps, LTE uses Orthogonal Frequency Division Multiplexing (OFDM) as its wireless access technology in a 20-MHz bandwidth. Also, LTE employs Adaptive Modulation & Coding (AMC) that adaptively determines a modulation scheme and a channel coding rate according to channel states of UEs.
FIG. 2 is a diagram illustrating a hierarchical structure of wireless protocols for an LTE system. With reference to FIG. 2, a description will now be made of the hierarchical structure of wireless protocols for the LTE system.
As illustrated in FIG. 2, wireless protocols of the LTE system include Packet Data Convergence Protocol (PDCP) 205 and 240, Radio Link Control (RLC) 210 and 235, and Medium Access Control (MAC) 215 and 230.
PDCP 205 and 240 take charge of an operation such as IP header compression/decompression, and RLC 210 and 235 reconfigure PDCP Packet Data Units (PDUs) in an appropriate size and perform an Automatic Repeat reQuest (ARQ) operation. In the following description, a packet output from a particular protocol entity will be referred to as a PDU of the protocol. MAC 215 and 230, which are connected to several RLC entities formed in one UE, perform an operation of multiplexing RLC PDUs to a MAC PDU, and demultiplexing RLC PDUs from a MAC PDU. Physical layers (PHY) 220 and 225 perform an operation of channel-coding and modulating upper layer data to make OFDM symbols and transmitting the OFDM symbols over a wireless channel, or an operation of demodulating and channel-decoding OFDM symbols received over a wireless channel and delivering the OFDM symbols to the upper layer.
While a UE is staying in one cell without moving to another cell, RLC entities ensure reliable data transmission/reception. On the contrary, when a UE moves to another cell, the UE removes the RLC entities used in the old cell and forms new RLC entities, so that a data loss may occur if continuity of the ARQ operation is not guaranteed. In a handover process, a source ENB forwards, to a target ENB, the PDCP SDUs for which it has not yet received transmission success/failure information (or ACK/NACK) at the RLC level, in order to prevent a possible data loss. However, since a link between ENBs is generally low in speed, it is preferable to reduce, if possible, the amount of data which is forwarded from the source ENB to the target ENB during handover.